The proposed research seeks to define the structural parameters which determine the spin-state of iron in hemoproteins. Numerous studies have indicated that the heme iron in ferric hemoproteins and their ligand substitution derivatives is characterized by temeature dependent spin-state equilibria. These studies have suggested that the equilibria between high-spin (S=5/2) and low-spin (S=1/2) states are a consequence of the effects of protein structure on the iron coordination rather than an intrinsic property of the iron coordination center. Recent studies in this laboratory have demonstrated that a spin-state equilibrium may be observed in a model heme ligand complex whose axial ligands correspond to those found in a hemoprotein derivative. This project seeks to characterize the spin-state properties of other model heme ligand complexes. Equilibrium constants and associated thermodynamic values will be compared to reported values for corresponding ligand complexes of hemoproteins. The results will provide a basis for evaluating the magnitude as well as the manner by which protein structure influences the spin-state properties of hemoproteins. The studies are relevant to an understanding of the diversity of functions of hemoproteins in biological systems.